Generally known conventional brushless motors consist of a rotor which has a plurality of field permanent magnets inserted into a rotor yoke made of laminated steel plates, and a stator which has a magnetic pole portion opposed to the outer peripheral face of the magnet portion of the rotor with a small gap therebetween.
This type of brushless motor is proposed, to detect a rotational position of the rotor, to adhere to an end face of the rotor a magnet piece to be detected for specifying a rotational position of the rotor, to dispose a magnetic sensor near the trajectory of rotation of the magnet piece to be detected, and to use the magnetic sensor to detect magnetism of the magnet piece to be detected, thereby detecting the rotational position of the rotor.
FIG. 37 shows a vertical sectional view of the above brushless motor having the magnet pieces. A brushless motor 51 has a pair of housing members 53, 54 fastened with bolts 52, and these housing members 53, 54 rotatably support a rotatable shaft 57 with ball bearings 55, 56. To the rotatable shaft 57, a rotor 58 is fixed, and one end of the rotatable shaft 57 is structured to protrude from the end face of the housing member 53 to externally transmit a rotary force of the rotor 58. A stator 59 is disposed around the rotor 58 and held between the housing members 53, 54.
The rotor 58 consists of a rotor yoke 50 which has many steel plates laminated, and a plurality of field permanent magnets 61 which are inserted into the rotor yoke 50. The stator 59 consists of a stator yoke 62 made of laminated steel plates, and stator coils 63 wound on the stator yoke 62. A part of the inner peripheral face of the stator yoke 62 forms a magnetic pole portion 59a of the stator, and the stator magnetic pole portion 59a is opposed to the outer peripheral face of a magnetic pole portion 58a of the rotor 58 with a small distance therebetween.
A magnet piece 64 to be detected is adhered to an end face 58b of the rotor 58. A magnetic sensor board 66 having a plurality of magnetic sensors 65 disposed is fixed to the housing member 53 near the trajectory of rotation of the magnet piece 64 to be detected.
In the above structure, when the rotor 58 of the brushless motor 51 rotates, the magnet piece 64 to be detected is also rotated and approached to the magnetic sensors 65 when it is rotated 360 degrees. The magnetic sensors 65 detect magnetism of the magnet piece 64 to be detected to detect the rotational position of the rotor 58. But, it is known that since this brushless motor 51 has a large distance between the field permanent magnets 61 and the stator magnet pole portion 59a, a magnetic flux is attracted in the rotating direction by an interaction with the stator magnetic pole portion 59a when rotating, and the position of a magnetic flux density peak point in an outside space of the rotor 58 does not agree with the actual rotational position of the rotor 58.
FIG. 38 shows a difference between a change of the magnetic flux density in the outside space of the rotor end face 58b of the brushless motor 51 and the rotational position of the rotor 58 detected by the magnet piece 64 to be detected. In FIG. 38, the horizontal axis shows a lapse of time, and the vertical axis shows the magnitude of an electric signal. Curve L1 shows a change of the magnetic flux density in the outside space of the rotor end face 58b, and kinked line L2 shows the rotational position of the rotor 58 detected by the magnet piece 64 to be detected. It is seen from the drawing that in a brushless motor having field permanent magnets in a rotor yoke and a relatively large distance between the field permanent magnets and a stator magnetic pole portion, a magnetic flux during rotation is attracted in the rotating direction by the stator magnetic pole portion, and the magnetic flux density (curve L1) forms a waveform advanced than the actual rotational position (kinked line L2) of the rotor. Specifically, the alternate long and short dash line indicates a state that the magnetic flux density (curve L1) is not advanced than the actual rotational position (kinked line L2) of the rotor, but the magnetic flux density (curve L1) indicated by the solid line is advanced by a time difference T at the position of point 0 of the electric signal than the magnetic flux density (curve L1) indicated by the alternate long and short dash line. This time difference T can be converted into a rotational angle of the rotor, and this rotational angle is equal to the displacement of the magnetic flux. And it is known that this displacement of the magnetic flux (hereinafter referred to as an angle of lead) increases substantially in proportion to the magnitude of a current applied to the motor.
Regarding the deviation of the position of the magnetic flux density peak point in the outside space of the rotor 58 from the actual rotational position of the rotor 58, when a Hall IC is used, a rotating speed is fixed at 1200 rpm, and torque is varied, the deviation of the peaks of the magnetic flux density outside of the rotor and the signal of the Hall IC obtained from the magnet to be detected at the maximum efficiency with respective torque is as follows.
______________________________________ Deviation of Hall IC and Torque peaks of magnetic flux Maximum (Kgm) density (electrical angle) efficiency (%) ______________________________________ 0.05 20.degree. .+-. 10.degree. 90 0.10 22.5.degree. .+-. 10.degree. 87 0.15 25.degree. .+-. 10.degree. 85 0.20 28.5.degree. .+-. 9.degree. 82 0.25 30.degree. .+-. 9.degree. 79 ______________________________________
As obvious from the above table, the peak point of the magnetic flux density in the space around the rotor 58 is advanced than the actual rotational position of the rotor 58. Further, this angle of lead is almost proportional to the motor torque, and an attaching error of the Hall IC to obtain the maximum efficiency is in a range of 20.degree. (.+-.10.degree.), but a tolerance becomes small as the torque increases, making it difficult to attach.
FIG. 39 is a magnetic sensor board of a three-phase four-pole brushless motor. In this case, a Hall IC was used for the magnetic sensor. A Hall IC 65 is one IC combining a function of detecting the direction of a magnetic field using the Hall effect and the function of an amplifier. When N pole is approached to above the Hall IC, output is about 5 (V), and when S pole, output is 0 (V). Therefore, one cycle of an electrical angle becomes N (5V) and S (0V), but since one cycle of a mechanical angle becomes N, S, N and S, it is known that the electrical angle to the mechanical angle is 2 to 1. (The electrical angle will be hereinafter referred to as the "electrical angle", and the mechanical angle will not be indicated.)
Generally, the three-phase four-pole brushless motor needs three Hall ICs 65 at intervals of 60 degrees peripherally on a circle having the same diameter (a distance R from the center axis is 23 mm in this example, which will be simply referred to as "R23" hereinafter), and they are soldered at intervals of 60 degrees on the magnetic sensor board 66. Furthermore, mounting holes 67 are formed at two locations of the magnetic sensor board 66 to fix to the housing member, and a pattern 69 is not formed on a peripheral portions 68 around the mounting holes 67. Lands 70 are disposed as connections to drive the Hall ICs 65 or to externally output a signal, and a through hole 71 is formed at the center of each land 70. A lead 72 is inserted in the through hole 71 from the back of the magnetic sensor board 66 and soldered on the land 70. The magnetic sensor board 66 has an outer periphery 73 to locate inside the coil 63 and an inner periphery 74 to locate outside the outer periphery of the housing bearing. And, an angle in the rotating direction is in a shape that a size for the mounting holes 67 is added to an arranging angle of 120 degrees for the three Hall ICs, so that the pattern is closely formed although the magnetic sensor board has a large shape.
Since the above conventional brushless motor detects only the rotational position of the rotor using the magnet piece to be detected, it has disadvantages that an angle of lead of the magnetic flux which varies depending on the motor current or motor torque cannot be detected, and when the magnetic pole portion of the stator is excited based on the detected signal, the stator magnetic pole portion which generates a rotary force most cannot be excited, and the motor efficiency is lowered.
On the other hand, it is considered to dispose the magnet piece to be detected or the magnetic sensor previously displaced in one direction assuming the angle of lead a magnetic flux, but this method cannot be applied to a bidirectionally rotatable brushless motor which is required to rotate the rotor in both directions.
While the above method detects the rotational position of the rotor yoke by the above magnetic sensor, there is a known sensorless brushless motor which detects the rotational position of the rotor by utilizing a back electromotive force to be generated on the stator side by the rotation of the rotor.
This sensorless brushless motor can detect the highest position of the magnetic flux density around the rotor, but has a disadvantage that an electric circuit is complicated because the back electromotive force generated on the stator side is detected.